The FIRST_VALUE and LAST_VALUE functions return values from the first and last rows of a window. This allows a query to access values from multiple rows at once, without the need for a self-join.
These two functions are different from the other window aggregate functions because they must be used with a window. Also, unlike the other window aggregate functions, these functions allow the IGNORE NULLS clause. If IGNORE NULLS is specified, the first or last non-NULL value of the desired expression is returned. Otherwise, the first or last value is returned.
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